When To Use Mega Steelix Over Mega Aggron In Competitive Pokemon

I have been playing Pokemon Showdown for a while now (I still play in gen 7 OU, because I really don't like the new gen 8). Every time I want a defensive steel type, my go to is Mega Steelix. Why? It has a super high defense stat, access to curse-gyro ball, which lowers it's speed while raising it's attack, making the next gyro ball twice as strong. Also, curse raises it's defensive stat, and you don't need to panic about special defense because that sucks already. That's one of the problems I most often find, it's not a reliable sweeper because of it's special defense. But it doesn't stop there, Mega Steelix also has rest-talk, meaning it uses rest, go's to sleep at the cost of replenishing it's health, but then sleep talk allows you to move while sleeping.

Mega Steelix was working really well for me, until I found my friend at school always uses Mega Aggron. They're advantages to him too. His base stat total is 20 points higher, and it has higher attack. However, it's only good move is heavy slam, which is really good don't get me wrong, but curse-gyro ball is better. It does get rest-talk though.

So the question arises, when should I use Mega Steelix and when should I use Mega Aggron? I mean, what team structure is one better?

TL;DR: Mega Aggron and Mega Steelix are both good choices that have good options when playing, though each offers a slightly different niche. However, rest-talk may not be the best set for them.

Advantages Mega Steelix has:

• Its ground typing means it has electric immunity, which allows it to ruin the momentum of volt switching Pokemon.
• As you mention, it has the powerful combination of curse and gyro ball.
• It also has access to STAB-boosted earthquake, which benefits from curse as well.

Advantages Mega Aggron has:

• Mega Aggron has the powerful ability Filter, which makes super-effective moves less damaging against it.
• Its pure steel typing means that, unlike Steelix, it has no weakness to water attacks, and also has a resistance against common ice attacks.
• A base attack of 140 (vs M-Steelix's 125) means it hits a little bit harder.

Essentially, the prime difference between M-Steelix and M-Aggron is that M-Steelix is a slow-to-start but powerful sweeper, that works best when you swap it in smartly to build its Curse boosts until it becomes unstoppable. M-Aggron is a bulky attacker that doesn't try to boost its stats but instead comes out of the gates swinging with powerful attacks like Earthquake and Heavy Slam.

One thing I will caution you against: while rest-talk isn't an impossible set to run, in most situations it's not going to be a set that shows off Mega Steelix's full potential. A slow Pokemon like Steelix doesn't like to allow opponents a chance to set up on it, and having a Pokemon switch in to your sleeping Steelix and ruin the health you just got back is devastating. Sleep Talk's unpredictable nature means you only have a one-in-three chance to actually attack your opponent with Gyro Ball, leaving your opponent an opening to bring in a special attacker that can ruin your Steelix's day and force you to swap out and lose all your stat-ups from Curse.

Steelix tends to benefit much more from having at least one support move like Stealth Rock or Toxic that leaves attackers frightened to switch in on it, and having a teammate with Wish who can pass it heals instead of trying to heal itself.